In modern homes, stoves are used to heat food and refrigerators are used to keep food cool. However, these devices are not readily portable and are not practical when the user is away from home, e.g., on a picnic or long trip.
An insulated thermos bottle is the most common device used to keep food, including liquids, either hot or cold. When a thermos is used, the food is preheated or cooled before it is placed in the thermos. Once in the thermos, the insulation of the thermos prevents heat exchange so that hot foods will retain their heat and cold foods will remain cool for a long period of time. However, food stored in a thermos or other insulated storage device, eventually reaches room temperature. Thus is it known to supplement the heat-retaining ability of a thermos by adding additional heat.
Additional heat can be applied to food by adding energy to it. This can be accomplished with electrical heating coils or by burning a combustible material (e.g., butane). Also, the radiant energy of the sun can be concentrated and reflected onto the container so as to warm it.
Cooling it typically performed by expansion of a gas, e.g., FREON. However, initially the gas must be compressed and this takes substantial energy and rather bulky compressor equipment. One alternative is to pack ice or some other frozen material in or around the thermos. This ice acts to supplement the low temperature of the food.
Typically, heat insulating thermos devices are designed for liquids, while cooling devices are designed for liquids and other foods. Also, heat and cold retaining devices are usually kept separate in order to prevent heat exchange between the two units.